1. Field of the Invention
The present invention relates to image processing apparatus and method for executing a color process by using spectral characteristics values, namely, spectral information.
2. Related Background Art
Hitherto, the following two methods are well known for a color designation or a color process in a color processing apparatus.
(a) The color designation and color process are executed by using calorimetric system signals such as RGB, CMYK, HVC, L*a*b*, YIQ, or the like, namely, a set of a small number of (such as 3 or 4) values as color information.
(b) The color designation and color process are executed by using a color sample displayed by display means such as CRT, LCD, silver-salt photograph, electrophotograph, printed matter, or the like as color information.
In color image, color information inherently includes spectral characteristics values such as spectral luminous intensity, spectral luminance, spectral reflectance, spectral transmittance, spectral absorption coefficient, and the like, namely, physical values depending on a wavelength of a light. Since each of those spectral characteristics values has a continuous infinite degree of freedom, a degree of freedom of the color designation and color process are very large.
The color information mentioned in the above item (a) merely uses a part of the degree of freedom of the information of the spectral characteristics values, namely, only 3 or 4 degree of freedom, so that the degree of freedom of the color designation and color process is limited.
The color information mentioned in the above item (b) has the problem as that of (a) in a sense that the display means uses only a part of the degree of freedom of the information of the spectral characteristics values like an example such that a display such as CRT, LCD, or the like uses RGB signals and an electrophotograph, a printed matter, or the like uses inks of four color of CMYK or of finite number of inks. Further in (b), since a color reproduction limit which is peculiar to each display means exists, namely, colors which cannot be displayed exist in the color sample, the degree of freedom of the color designation and color process is further small.
A specific example showing such a shortage of the degree of freedom will now be described hereinbelow.
FIG. 8 is a diagram showing the spectral characteristics values showing the colors of a printed matter. The printed matter is irradiated by a light i.sub.0 (.lambda.) from a light source and outputs a light of i(.lambda.) to an observer. In this instance, i.sub.0 (.lambda.) and i(.lambda.) denote spectral irradiation luminances.
A spectral reflectance R(.lambda.) of the printed matter is expressed by using the spectral irradiation luminances by the following equation. EQU i(.lambda.)=R(.lambda.)i.sub.0 (.lambda.)
For a spectral characteristics value F(.lambda.) of such a spectral irradiation luminance, spectral reflectance, or the like, RGB values by an rgb calorimetric system of the CIE (1931) are calculated by the following conversion equations by using color functions r, g, b of rgb or the like in FIG. 9. ##EQU1##
FIG. 10 is a diagram showing the spectral characteristics values when the same printed matter is irradiated by two kinds of light sources.
A light source 1 is called equal energy spectral white. A light source 2 is obtained by color matching the light source 1 by monochromatic spectral lights of three stimulus values such as 435.8 nm, 546.1 nm, and 700 nm of the rgb system determined by the CIE.
It is now assumed in the present specific example that the printed matter reflects the lights of wavelengths below 690 nm and absorbs the lights of wavelengths above 690 nm.
The spectral characteristics values of the printed matter irradiated by the light sources 1 and 2 are as shown in the diagram and the lights whose wavelengths are 690 nm or longer are lost, respectively.
FIG. 11 is a diagram showing the same phenomenon as that of FIG. 10 by RGB values by the above-mentioned conversion equations.
From FIGS. 10 and 11, it will be understood that there are differences of the observed values between the light sources 1 and 2 with respect to the spectral characteristics values and the RGB values.
When an attention is paid only to the display by the RGB values in FIG. 11, however, the two systems of the light sources 1 and 2 are substantially the same system with respect to both of the light source and the printed matter. The result such that the different RGB values occur as observed values from quite the same system as mentioned above shows that it is impossible to express the color of such a system by the RGB values and the degree of freedom for the color designation is insufficient.
A fact that, hitherto, a color deviation occurring by a light source change in case of using the light source including bright lines such as a fluorescent lamp cannot be sufficiently adjusted will now be explained hereinbelow as a specific example.
FIG. 12 is a diagram showing spectral distributions. A spectral distribution 101 shows a color of a reflection matter; a spectral distribution 102 a color of the light source 1; and a spectral distribution 103 a color of a reflection matter 101 irradiated by the light source 102.
FIG. 13 is a diagram showing spectral distributions when the light source 1 used in FIG. 12 is changed to the light source 2. The spectral distribution 101 shows the color of the same reflection matter as that in FIG. 12; a spectral distribution 202 a color of the light source 2; and a spectral distribution 203 a color of the reflection matter irradiated by a light source 202.
The spectral distribution of the fluorescent lamp which is generally used includes the bright lines of 389 nm, 405 nm, 408 nm, 436 nm, 492 nm, 546 nm, 578 nm, and 691 nm.
In order not to lose the generality, two bright lines for the light source 1 and three bright lines for the light source 2 are selected and combined from among the bright lines included in the fluorescent lamp and (greenish) while light is used as both of the light sources 1 and 2. The XYZ values of the light sources 1 and 2 are the same (100, 139, 122).
For the two light sources 1 and 2 having the same XYZ values as mentioned above, the chromaticity values of the three-value type such as rgb values, L*a*b* values, or the like which are converted and calculated from the XYZ values are naturally the same.
Under such a condition, when a calculation to adjust the color deviation in association with the change of the light source is performed, since there is no change with respect to the three chromaticity values, it is judged that there is no color deviation.
The color deviation, however, actually exists and the colors which are actually obtained are light green blue (XYZ values: 81, 116, 109) and dark red purple (XYZ values: 8, 2, 24) which are different in hue, lightness, and chroma saturation as shown in the spectral distributions 103 and 203. Practically, it is necessary to adjust such a color deviation, namely, from light green blue to dark red purple.
As mentioned above, the adjustment of the color deviation in association with the light source change cannot be performed at a sufficient accuracy. This is a big problem also from a viewpoint such that a color matching system is built in the environment of different light sources. Particularly, when considering a present situation such that in many cases, color printed matters and pictures are appreciated and observed under the fluorescent lamp, it is a very serious problem.